As consumer demand for high data rate applications, such as streaming video, expands, technology providers are forced to adopt new technologies to provide the necessary bandwidth. Multiple Input Multiple Output (“MIMO”) is an advanced radio system that employs multiple transmit antennas and multiple receive antennas to simultaneously transmit multiple parallel data streams. Relative to previous wireless technologies, MIMO enables substantial gains in both system capacity and transmission reliability without requiring an increase in frequency resources. MIMO systems exploit differences in the paths between transmit and receive antennas to increase data throughput and diversity.
MIMO systems can be used to improve transmission reliability by transmitting multiple signals which represent the same data over the variant wireless channels, thus increasing the chances that the data can be recovered successfully. Diversity gain available from MIMO systems is equal to the product of the number of transmit antennas and the number of receive antennas. Space-time codes have been proposed as a means of attaining diversity in MIMO systems in the absence of channel state information (“CSI”), i.e. the channel transfer function, at the transmitter.
Transmit beamforming, also known as closed loop beamforming, is an alternative technique for achieving spatial diversity in MIMO systems. When compared with space-time coding, transmit beamforming requires a less complex receiver and provides enhanced array gain. Transmit beamforming relies on the transmitter having knowledge of the channel's characteristics, channel state information, and in order to avoid assumptions regarding reciprocal up-link and down-link channels, CSI may be provided to the transmitter by the receiver via a feedback channel.
The IEEE 802.11n wireless networking standard specifies multiple techniques for performing transmit beamforming. The techniques differ as to whether the burden of beamforming complexity is placed at the transmitter or the receiver, and as to the quantity of information that must be fed back to the transmitter. Limited feedback resources may result in a compromise between supplying the quantity of information necessary to completely characterize the channel and supplying a lesser quantity of information that may result in a higher complexity receiver and/or negate some of the benefits of transmit beamforming. Thus, an improved technique is needed that strikes a better balance between feedback overhead and transmitter/receiver complexity.